Flat Roof and Solar Energy Design

Few homes are designed with a flat or marginally sloped roof, although many business facilities including retail have a flat roof. The term flat doesn’t mean perfectly horizontal, and generally the roof surfaces are built with minor angles to make drainage more effective. Some may slope slightly in one direction. Others may have a more complex set of minor sloped surfaces for drainage. For the purposes of solar energy system design, they are all considered flat.

Designing for a Flat Roof

For larger solar energy systems, often 100 kW and greater, economies of scale apply that can reduce the total cost for each unit of solar power produced. Longer strings and rows of solar panels may be installed. Larger capacity power inverters could be applied to these larger arrays, resulting in fewer over components.

In Massachusetts and Rhode Island, most flat roof structures allow for more solar panel mounting options. One of the most preferred methods is a ballasted mounting, where the solar panels and racking are interconnected and weighted to stay in contact with the roof. No direct mechanical fastening is required. Mechanical fastening and a combination of mechanical and interconnect fastening is used where needed, too.

With direct mechanical fastening, smaller sections of solar panels and racking can be independently affixed to the roof support structure. A hybrid version allows for more interconnection of solar arrays similar to a ballasted system, with few attachments to the roof supports.

Flat Roof Solar Power Design Considerations

Flat roof structures open up a number of solar energy system design options. Often there is more available surface for mounting solar panels, and the challenges presented by more complex pitched roof designs is avoided. Some general considerations include the following:

  • As these kinds of facilities tend to be larger, solar panel mounting and racking can be more economically designed to meet a target tilt angle. This allows for a slightly more efficient year-round solar power production when compared to pitched roof mounting.
  • Optionally, solar panels can be mounted with just enough tilt to allow for sufficient water draining and snow melting.
  • Increased roof area may also facilitate proper solar panel orientation towards the south to gain marginal solar power production efficiency. This also makes mounting larger solar arrays in a east-west orientation easier. See {link to east-west mounting page.}
  • Roof membranes generally require prescribed, routine maintenance. Some may be coated to improve weather resistance or to enhance reflection for energy efficiency. Prior to installing a solar energy system, roofing experts are often asked to review and confirm the membrane and surface readiness. This avoids the need for removing solar panels for maintenance access when it could have been avoided.
  • Many flat roof structures are more accessible for staging the solar panels and hardware for larger systems. On-roof installation activities are typically more smooth and efficient, too.